Some Stroboscopic Spinoffs in Photographic Technology

Andrew Davidhazy
School of Photographic Arts and Sciences
Imaging and Photographic Technology
Rochester Institute of Technology


The potential of the high speed electronic flash and the stroboscope to significantly contribute to the fields of science and technology can not be over emphasized. Important applications for these instruments and applied techniques have been described by Dr. Harold Edgerton and elaborated upon by numerous other workers.

In the School of Photographic Arts and Sciences at the Rochester Institute of Technology, fundamental principles and a variety of technical applications are a part of the education of a group of students pursuing a Bachelor of Science degree in the area of Imaging and Photographic Technology.

Graduates of this program are prepared for employment in a variety of positions within a broad field encompassing industrial, scientific, engineering, and technical photography as well as technical sales support activities related to diverse photographic and other imaging specialties. Within the context of their required coursework, students are required to complete a High-speed Photography course and may elect other courses in applied photoinstrumentation. The subject matter often is flavored with references to pioneering work developed by Dr. Edgerton.

This paper presents a summary of some of the projects included in these courses. They are presented here to not only gratefully acknowledge the direct connection to Dr. Edgerton's pioneering work but also to reemphasize the usefulness of specific electronic flash and stroboscopic analysis techniques.


_Moving Film Stroboscopy_.

In traditional stroboscopic analysis of human or mechanical movements, a stroboscope is set to repeat at a known frequency. The subject, placed against a black background, executes its motion while the shutter of the camera remains open for a brief period of time. This results in a series of overlapping exposures from which it is possible to visualize and measure the characteristics of the action.

Drawbacks of this technique are that stationary parts of the subject are multiple exposed building up high densities and that the recording time is limited by the eventual obscuration of motion detail by superimposition of images over previously exposed film areas. A number of techniques for overcoming these problems will be discussed. A simplified system for producing stroboscopic sequences which adequately depict subject motion along with the added benefit of extended recording time will be illustrated. The basis for extending the recording time is provided by rewinding unexposed film back into a 35mm camera's supply spool while keeping the shutter of the camera open. The sequential flashes of the stroboscope illuminate the subject at various positions in its action and these are therefore impressed onto the moving film leaving an extended "track" of subject motion versus time.

_Combined tungsten/stroboscopic images for greater motion certainty_.

Stroboscopic motion analysis often suffers from the fact that there is no information recorded by the photosensitive material during the "dark" portions of the exposure. This could lead to misinterpretation of the results if the frequency of operation is the parameter which needs to be determined.

To overcome this limitation, students perform the stroboscopic analysis of a sewing machine movement with an added feature. To insure that they do not miscalculate the stitching frequency they attach a small incandescent lamp to the needle assembly. This lamp describes a continuous record, effectively a streak record, of the lamp position vs. time. Since the stroboscope provides the time base and the instantaneous view of the position of the various machine parts, motion analysis can be performed thoroughly.

_Study of radio tower stroboscopes with an improvised streak camera_.

The behavior of another "Edgerton light" was studied in a project where the simultaneity and flashing frequency of a strobe illumination system installed in a local radio tower had to be determined. The power of the streak camera as an analytical tool was again demonstrated. For this project the camera used as an inexpensive ($10) plastic bodied 35mm camera modified to accept Leica screw mount lenses and fitted with a motorized film advance system salvaged from an old Pentax winder attachment.

The study was conducted at night and the moving film clearly recorded the timing pattern as well as the flashing frequency of each stroboscopic warning beacon. The frequency of a sodium vapor street lamp was used to provide the time base with which the timing of the tower strobes was calculated.

_Rotating drum combined with a stroboscope for sequences of short duration_.

A simple rotating drum camera, constructed out of plywood, a sewing machine rotor, a blower fan, a lens flange and a lens, was used to study the operating characteristics of the unique F280 extended duration flash manufactured by the Olympus Corporation.

This electronic flash can operate either in the conventional mode or in the extended duration mode. In the former case it can only be used up to the X sync speed of the camera. In the latter case, the flash allows even the top shutter speed of 1/2000 second to be used. This study determined that the flash is transformed into a high repetition stroboscope when set in the FP mode. The operating frequency, the duration and the evenness of illumination across the film plane were visualized and measured.

_Timing with a rotating mirror streak camera_.

While the small drum camera is used to measure triggering delays inherent in light and radio slave flash triggers as used in typical studio situations, its time resolution is not high enough to resolve time delays in the microsecond range. For streak studies in this time frame the Department has available a surplus Cordin S-4 rotating mirror streak camera. Since the associated timing and synchronization accessories are not available, students have to use improvised and adapted instrumentation to extract useful data with this instrument. They monitor the mirror rotation rate with a General Radio Strobotac and visually monitor the mirror position at the time of the event by noting the position of markers attached to the mirror shaft and illuminated by the event. This streak camera will be used during the Spring quarter to study the "blue flash" characteristic of variable power electronic flashes operating at short durations.

These project summaries are an indication of the fact that Dr. Edgerton's leadership and influence are alive and well at RIT. We are deeply indebted to him for making the high speed electronic flash and the stroboscope such popular and useful tools for scientific and engineering study.